1. Field of the Invention
The present invention relates to a method of measuring an angular direction of null ("null angle") in a radiation pattern of a monopulse antenna, and an apparatus therefor.
2. Prior Art
FIGS. 1a-1c show a method according to a prior art which is adapted to measure the null angle of a monopulse antenna in a difference pattern. In these figures, the reference numeral 100a designates an aperture of a monopulse antenna 100; and 100b a mechanical boresight axis of the monopulse antenna. The monopulse antenna 100 is supported by a mechanical means rotatable in the horizontal plane. On the other hand, a transmission antenna is so arranged that the maximum value in its radiation pattern E.sub.t is oriented toward the aperture center of the monopulse antenna 100.
As the monopulse antenna 100 is rotated in the horizontal plane, a reception level in the difference pattern of the monopulse antenna 100 will vary as indicated by the arrows in FIGS. 1a-1c. Thus, the null angle .delta. in the difference pattern can be measured by serially detecting the amplitudes of difference signals E.sub..DELTA. output from the difference signal output terminal of the monopulse antenna 100 as a function of rotational angles .theta..sub.1, .theta..sub.2 and .theta..sub.3 of the monopulse antenna 100.
More specifically, the absolute value of the difference signal output becomes minimum in the angular direction .delta. and, then, the angle .delta. can be understood as indicating the null angle.
A sum signal E.sub..SIGMA. output at a sum signal output terminal of the monopulse antenna 100 can also be measured in the same manner as described above. By dividing the difference signal E.sub..DELTA. by the sum signal E.sub..SIGMA., the null angle of the difference signal vs. sum signal pattern can be specified.
A rotational angle of the monopulse antenna 100 can normally be detected by a synchronous signal from a synchronous transmitter or a digital signal from a shaft encoder provided on a rotary apparatus, for example, a rotator of the monopulse antenna 100.
It is to be noted, however, that the degree of accuracy in measuring the null angle depends largely on mechanical parameters such as the degree of accuracy in positioning a rotator and the repeatability of a rotational angle and the like. Furthermore, setting a rotator to a predetermined angle results in further time being taken to carry out a measurement. It has been found out that the prior art method is not suitable for conducting measurements, with a high degree of accuracy, the null angle in the radiation pattern of an antenna adapted to perform a minute beam scanning such as an electrically scanning monopulse antenna.